[syzbot] [fuse?] KASAN: slab-out-of-bounds Write in fuse_dev_do_write

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: cf6fc5eefc5b Merge tag 's390-6.17-3' of git://git.kernel.o..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=15348c42580000
kernel config: https://syzkaller.appspot.com/x/.config?x=b7511150b112b9c3
dashboard link: https://syzkaller.appspot.com/bug?extid=2d215d165f9354b9c4ea
compiler: gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=147a5062580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=139caa34580000

Downloadable assets:
disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-cf6fc5ee.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/a27518272e48/vmlinux-cf6fc5ee.xz
kernel image: https://storage.googleapis.com/syzbot-assets/cf3f4cc06dfd/bzImage-cf6fc5ee.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+2d215d...@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: slab-out-of-bounds in fuse_retrieve fs/fuse/dev.c:1911 [inline]
BUG: KASAN: slab-out-of-bounds in fuse_notify_retrieve fs/fuse/dev.c:1959 [inline]
BUG: KASAN: slab-out-of-bounds in fuse_notify fs/fuse/dev.c:2067 [inline]
BUG: KASAN: slab-out-of-bounds in fuse_dev_do_write+0x308b/0x3420 fs/fuse/dev.c:2158
Write of size 4 at addr ffff88803b8fc6dc by task syz.0.17/6135

CPU: 0 UID: 0 PID: 6135 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:94 [inline]
dump_stack_lvl+0x116/0x1f0 lib/dump_stack.c:120
print_address_description mm/kasan/report.c:378 [inline]
print_report+0xcd/0x630 mm/kasan/report.c:482
kasan_report+0xe0/0x110 mm/kasan/report.c:595
fuse_retrieve fs/fuse/dev.c:1911 [inline]
fuse_notify_retrieve fs/fuse/dev.c:1959 [inline]
fuse_notify fs/fuse/dev.c:2067 [inline]
fuse_dev_do_write+0x308b/0x3420 fs/fuse/dev.c:2158
fuse_dev_write+0x155/0x1e0 fs/fuse/dev.c:2242
new_sync_write fs/read_write.c:593 [inline]
vfs_write+0x7d3/0x11d0 fs/read_write.c:686
ksys_write+0x12a/0x250 fs/read_write.c:738
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xcd/0x4c0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f440eb8ebe9
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f440f9e1038 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007f440edb5fa0 RCX: 00007f440eb8ebe9
RDX: 0000000000000030 RSI: 0000200000000140 RDI: 0000000000000004
RBP: 00007f440ec11e19 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f440edb6038 R14: 00007f440edb5fa0 R15: 00007ffddcd08da8
</TASK>

Allocated by task 6135:
kasan_save_stack+0x33/0x60 mm/kasan/common.c:47
kasan_save_track+0x14/0x30 mm/kasan/common.c:68
poison_kmalloc_redzone mm/kasan/common.c:388 [inline]
__kasan_kmalloc+0xaa/0xb0 mm/kasan/common.c:405
kasan_kmalloc include/linux/kasan.h:260 [inline]
__do_kmalloc_node mm/slub.c:4365 [inline]
__kmalloc_noprof+0x223/0x510 mm/slub.c:4377
kmalloc_noprof include/linux/slab.h:909 [inline]
kzalloc_noprof include/linux/slab.h:1039 [inline]
fuse_retrieve fs/fuse/dev.c:1879 [inline]
fuse_notify_retrieve fs/fuse/dev.c:1959 [inline]
fuse_notify fs/fuse/dev.c:2067 [inline]
fuse_dev_do_write+0x1c50/0x3420 fs/fuse/dev.c:2158
fuse_dev_write+0x155/0x1e0 fs/fuse/dev.c:2242
new_sync_write fs/read_write.c:593 [inline]
vfs_write+0x7d3/0x11d0 fs/read_write.c:686
ksys_write+0x12a/0x250 fs/read_write.c:738
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xcd/0x4c0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

The buggy address belongs to the object at ffff88803b8fc600
which belongs to the cache kmalloc-256 of size 256
The buggy address is located 4 bytes to the right of
allocated 216-byte region [ffff88803b8fc600, ffff88803b8fc6d8)

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x3b8fc
head: order:1 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
flags: 0xfff00000000040(head|node=0|zone=1|lastcpupid=0x7ff)
page_type: f5(slab)
raw: 00fff00000000040 ffff88801b842b40 dead000000000122 0000000000000000
raw: 0000000000000000 0000000080100010 00000000f5000000 0000000000000000
head: 00fff00000000040 ffff88801b842b40 dead000000000122 0000000000000000
head: 0000000000000000 0000000080100010 00000000f5000000 0000000000000000
head: 00fff00000000001 ffffea0000ee3f01 00000000ffffffff 00000000ffffffff
head: ffffffffffffffff 0000000000000000 00000000ffffffff 0000000000000002
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 1, migratetype Unmovable, gfp_mask 0x52820(GFP_ATOMIC|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP), pid 1151, tgid 1151 (kworker/u32:8), ts 61845903757, free_ts 55626037651
set_page_owner include/linux/page_owner.h:32 [inline]
post_alloc_hook+0x1c0/0x230 mm/page_alloc.c:1851
prep_new_page mm/page_alloc.c:1859 [inline]
get_page_from_freelist+0x132b/0x38e0 mm/page_alloc.c:3858
__alloc_frozen_pages_noprof+0x261/0x23f0 mm/page_alloc.c:5148
alloc_pages_mpol+0x1fb/0x550 mm/mempolicy.c:2416
alloc_slab_page mm/slub.c:2487 [inline]
allocate_slab mm/slub.c:2655 [inline]
new_slab+0x247/0x330 mm/slub.c:2709
___slab_alloc+0xcf2/0x1740 mm/slub.c:3891
__slab_alloc.constprop.0+0x56/0xb0 mm/slub.c:3981
__slab_alloc_node mm/slub.c:4056 [inline]
slab_alloc_node mm/slub.c:4217 [inline]
__kmalloc_cache_noprof+0xfb/0x3e0 mm/slub.c:4391
kmalloc_noprof include/linux/slab.h:905 [inline]
kzalloc_noprof include/linux/slab.h:1039 [inline]
batadv_bla_get_backbone_gw+0xbe/0xc40 net/batman-adv/bridge_loop_avoidance.c:508
batadv_bla_update_own_backbone_gw.isra.0+0x4e/0x170 net/batman-adv/bridge_loop_avoidance.c:571
batadv_bla_tx+0x144/0x21b0 net/batman-adv/bridge_loop_avoidance.c:2105
batadv_interface_tx+0x5e7/0x1b80 net/batman-adv/mesh-interface.c:227
__netdev_start_xmit include/linux/netdevice.h:5222 [inline]
netdev_start_xmit include/linux/netdevice.h:5231 [inline]
xmit_one net/core/dev.c:3839 [inline]
dev_hard_start_xmit+0x94/0x740 net/core/dev.c:3855
__dev_queue_xmit+0xa46/0x4490 net/core/dev.c:4725
dev_queue_xmit include/linux/netdevice.h:3361 [inline]
neigh_resolve_output net/core/neighbour.c:1595 [inline]
neigh_resolve_output+0x53a/0x940 net/core/neighbour.c:1575
neigh_output include/net/neighbour.h:547 [inline]
ip6_finish_output2+0xaee/0x2020 net/ipv6/ip6_output.c:141
page last free pid 0 tgid 0 stack trace:
reset_page_owner include/linux/page_owner.h:25 [inline]
free_pages_prepare mm/page_alloc.c:1395 [inline]
__free_frozen_pages+0x7d5/0x10f0 mm/page_alloc.c:2895
rcu_do_batch kernel/rcu/tree.c:2605 [inline]
rcu_core+0x799/0x1530 kernel/rcu/tree.c:2861
handle_softirqs+0x216/0x8e0 kernel/softirq.c:579
__do_softirq kernel/softirq.c:613 [inline]
invoke_softirq kernel/softirq.c:453 [inline]
__irq_exit_rcu+0x109/0x170 kernel/softirq.c:680
irq_exit_rcu+0x9/0x30 kernel/softirq.c:696
instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1050 [inline]
sysvec_apic_timer_interrupt+0xa4/0xc0 arch/x86/kernel/apic/apic.c:1050
asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:702

Memory state around the buggy address:
ffff88803b8fc580: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff88803b8fc600: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>ffff88803b8fc680: 00 00 00 00 00 00 00 00 00 00 00 fc fc fc fc fc
^
ffff88803b8fc700: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff88803b8fc780: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
==================================================================


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzk...@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want syzbot to run the reproducer, reply with:
#syz test: git://repo/address.git branch-or-commit-hash
If you attach or paste a git patch, syzbot will apply it before testing.

If you want to overwrite report's subsystems, reply with:
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If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup

[Edward Adam Davis]

#syz test

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index e80cd8f2c049..5150aa25e64b 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -1893,7 +1893,7 @@ static int fuse_retrieve(struct fuse_mount *fm, struct inode *inode,

index = outarg->offset >> PAGE_SHIFT;

- while (num) {
+ while (num && ap->num_folios < num_pages) {
struct folio *folio;
unsigned int folio_offset;
unsigned int nr_bytes;

[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-by: syzbot+2d215d...@syzkaller.appspotmail.com
Tested-by: syzbot+2d215d...@syzkaller.appspotmail.com

Tested on:

commit: 6debb690 Merge tag 'drm-fixes-2025-08-23-1' of https:/..
git tree: upstream

kernel config: https://syzkaller.appspot.com/x/.config?x=b7511150b112b9c3
dashboard link: https://syzkaller.appspot.com/bug?extid=2d215d165f9354b9c4ea
compiler: gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40

patch: https://syzkaller.appspot.com/x/patch.diff?x=113a8c42580000

Note: testing is done by a robot and is best-effort only.

[Joanne Koong]

Thanks for the report. I think the issue arises in cases where the
calculation for num_pages has to get rounded down to fc->max_pages and
a non-zero offset is passed in.

> ---
> This report is generated by a bot. It may contain errors.
> See https://goo.gl/tpsmEJ for more information about syzbot.
> syzbot engineers can be reached at syzk...@googlegroups.com.
>
> syzbot will keep track of this issue. See:
> https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
>
> If the report is already addressed, let syzbot know by replying with:
> #syz fix: exact-commit-title
>
> If you want syzbot to run the reproducer, reply with:
> #syz test: git://repo/address.git branch-or-commit-hash
> If you attach or paste a git patch, syzbot will apply it before testing.

#syz test: upstream cf6fc5eefc5bbbbff92a085039ff74cdbd065c29

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index e80cd8f2c049..e84e05de9cdb 100644

--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -1893,7 +1893,7 @@ static int fuse_retrieve(struct fuse_mount *fm,
struct inode *inode,

index = outarg->offset >> PAGE_SHIFT;

- while (num) {

+ while (num && num_pages) {

struct folio *folio;
unsigned int folio_offset;
unsigned int nr_bytes;

@@ -1914,6 +1914,7 @@ static int fuse_retrieve(struct fuse_mount *fm,
struct inode *inode,

offset = 0;
num -= nr_bytes;
+ num_pages -= nr_pages;
total_len += nr_bytes;
index += nr_pages;
}

[syzbot]

Hello,

syzbot tried to test the proposed patch but the build/boot failed:

failed to apply patch:
checking file fs/fuse/dev.c
patch: **** unexpected end of file in patch



Tested on:

commit: cf6fc5ee Merge tag 's390-6.17-3' of git://git.kernel.o..
git tree: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git

kernel config: https://syzkaller.appspot.com/x/.config?x=b7511150b112b9c3
dashboard link: https://syzkaller.appspot.com/bug?extid=2d215d165f9354b9c4ea
compiler:

patch: https://syzkaller.appspot.com/x/patch.diff?x=116eaa34580000

[Joanne Koong]

#syz test: upstream cf6fc5eefc5bbbbff92a085039ff74cdbd065c29

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index e80cd8f2c049..e84e05de9cdb 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -1893,7 +1893,7 @@ static int fuse_retrieve(struct fuse_mount *fm,
struct inode *inode,

index = outarg->offset >> PAGE_SHIFT;

- while (num) {
+ while (num && num_pages) {
struct folio *folio;
unsigned int folio_offset;
unsigned int nr_bytes;
@@ -1914,6 +1914,7 @@ static int fuse_retrieve(struct fuse_mount *fm,
struct inode *inode,

offset = 0;
num -= nr_bytes;
+ num_pages -= nr_pages;
total_len += nr_bytes;
index += nr_pages;
}

--
2.47.3

[syzbot]

Hello,

syzbot tried to test the proposed patch but the build/boot failed:

failed to apply patch:
checking file fs/fuse/dev.c
patch: **** unexpected end of file in patch



Tested on:

commit: cf6fc5ee Merge tag 's390-6.17-3' of git://git.kernel.o..
git tree: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
kernel config: https://syzkaller.appspot.com/x/.config?x=b7511150b112b9c3
dashboard link: https://syzkaller.appspot.com/bug?extid=2d215d165f9354b9c4ea
compiler:

patch: https://syzkaller.appspot.com/x/patch.diff?x=12393fa2580000

[Edward Adam Davis]

syz reported a slab-out-of-bounds Write in fuse_dev_do_write.

Using the number of bytes alone as the termination condition in a loop
can prematurely exhaust the allocated memory if the incremented byte count
is less than PAGE_SIZE.

Add a loop termination condition to prevent overruns.

Fixes: 3568a9569326 ("fuse: support large folios for retrieves")
Reported-by: syzbot+2d215d...@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=2d215d165f9354b9c4ea
Tested-by: syzbot+2d215d...@syzkaller.appspotmail.com
Signed-off-by: Edward Adam Davis <ead...@qq.com>
---
fs/fuse/dev.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index e80cd8f2c049..5150aa25e64b 100644

--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -1893,7 +1893,7 @@ static int fuse_retrieve(struct fuse_mount *fm, struct inode *inode,

index = outarg->offset >> PAGE_SHIFT;

- while (num) {

+ while (num && ap->num_folios < num_pages) {

struct folio *folio;
unsigned int folio_offset;
unsigned int nr_bytes;

--
2.43.0

[Joanne Koong]

On Fri, Aug 22, 2025 at 6:17 PM Edward Adam Davis <ead...@qq.com> wrote:
>
> syz reported a slab-out-of-bounds Write in fuse_dev_do_write.
>
> Using the number of bytes alone as the termination condition in a loop
> can prematurely exhaust the allocated memory if the incremented byte count
> is less than PAGE_SIZE.

I don't think the last part of this is quite right. It's fine if the
incremented byte count is less than PAGE_SIZE (which will always be
the case if there's an offset). We only run into this issue when the
number of bytes to retrieve gets truncated by fc->max_pages as the
upper bound and there's an offset.

>
> Add a loop termination condition to prevent overruns.
>
> Fixes: 3568a9569326 ("fuse: support large folios for retrieves")
> Reported-by: syzbot+2d215d...@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=2d215d165f9354b9c4ea
> Tested-by: syzbot+2d215d...@syzkaller.appspotmail.com
> Signed-off-by: Edward Adam Davis <ead...@qq.com>
> ---
> fs/fuse/dev.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
> index e80cd8f2c049..5150aa25e64b 100644
> --- a/fs/fuse/dev.c
> +++ b/fs/fuse/dev.c
> @@ -1893,7 +1893,7 @@ static int fuse_retrieve(struct fuse_mount *fm, struct inode *inode,
>
> index = outarg->offset >> PAGE_SHIFT;
>
> - while (num) {
> + while (num && ap->num_folios < num_pages) {
> struct folio *folio;
> unsigned int folio_offset;
> unsigned int nr_bytes;
> --
> 2.43.0

Thanks for the fix.

Reviewed-by: Joanne Koong <joanne...@gmail.com>

[Edward Adam Davis]

syz reported a slab-out-of-bounds Write in fuse_dev_do_write.

When the number of bytes to be retrieved is truncated to the upper limit
by fc->max_pages and there is an offset, the oob is triggered.

Add a loop termination condition to prevent overruns.

Fixes: 3568a9569326 ("fuse: support large folios for retrieves")
Reported-by: syzbot+2d215d...@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=2d215d165f9354b9c4ea
Tested-by: syzbot+2d215d...@syzkaller.appspotmail.com
Signed-off-by: Edward Adam Davis <ead...@qq.com>

Reviewed-by: Joanne Koong <joanne...@gmail.com>
---
V1 -> V2: update root cause

[Miklos Szeredi]

On Wed, 27 Aug 2025 at 03:46, Edward Adam Davis <ead...@qq.com> wrote:
>
> syz reported a slab-out-of-bounds Write in fuse_dev_do_write.
>
> When the number of bytes to be retrieved is truncated to the upper limit
> by fc->max_pages and there is an offset, the oob is triggered.
>
> Add a loop termination condition to prevent overruns.

Applied, thanks.

Miklos